The BGS Lexicon of Named Rock Units — Result Details

Bagshot Formation
Computer Code: BGS Preferred Map Code: BgB
Status Code: Full
Age range: Ypresian Age (GY) — Ypresian Age (GY)
Lithological Description: Most of the Bagshot Formation is composed of pale yellow-brown to pale grey or white, locally orange or crimson, fine- to coarse-grained sand that is frequently micaceous and locally clayey, with sparse glauconite and sparse seams of gravel. The sands are commonly cross-bedded but some are laminated. Thin beds and lenses of laminated pale grey to white sandy or silty clay or clay (‘pipe-clay’) occur sporadically, becoming thicker towards the top of the formation. A thick clay bed, the Swinley Clay Member, is included at the top. In places, there is a basal bed of gravelly coarse-grained sand. There is a sparse fossil fauna of mostly indeterminate marine molluscs, with some indistinct plant remains (Curry, 1958; Hawkins, 1954), but most organic material has been destroyed by oxidation or dissolution. The higher part of the interval in the Bracknell Borehole (SU86NE42) [SU 8888 6547] is predominantly bioturbated, with frequent Ophiomorpha burrows (King, unpublished report to BGS, 1996; (Ellison and Williamson, 1999, fig. 6). As mapped by BGS, in places the lower part of the Bagshot Formation probably includes an interval of bioturbated sandy clay, silt and fine-grained sand overlying a unit of fine- to coarse-grained sand (Bracknell Member of King, in prep.), both of which are placed in the topmost London Clay Formation by King (in prep.). A temporary section near the M3 in Surrey, described by Goldring et al. (1978), exposed interlayered sands and muds, fine sands, channel-fill sands and intraformational (mainly mud clast) conglomerates. The facies showed rapid lateral and vertical changes in grain size and bed form and a restricted suite of trace fossils including Ophiomorpha nodosa and Arenicolites sp. The part of the formation that this section represents is not known.
Definition of Lower Boundary: The base of the Bagshot Formation is marked by an erosional surface marking a change from clay, silt and fine-grained sand of the Claygate Member (London Clay Formation) to thick-bedded, pale-coloured, fine-grained sands, with a basal fine gravelly sand developed in places. Locally, erosion has removed the topmost parts of the Claygate Member.
Definition of Upper Boundary: The top of the Bagshot Formation is an erosional surface at the top of the Swinley Clay Member, marking a change to the glauconitic sands of the lower part of the Windlesham Formation. In the outliers in the north of London, and to the east, the Bagshot Formation is the youngest part of the Palaeogene succession and the upper part has been eroded.
Thickness: Up to about 45m to the south-west of London, where overlain by the Windlesham Formation.
Geographical Limits: The Bagshot Formation is found in the axial part of the London Basin.
Parent Unit: Bracklesham Group (BRB)
Previous Name(s): Bagshot Sands (-2602)
Bagshot Beds (-693)
Lower Bagshot Beds (-5080)
Bagshot Beds [Obsolete Name And Code: Use BGS] (BGB)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Heathland, near Bagshot, Frimley, Pirbright, Surrey. 
Reference(s):
GOLDRING, R, BOSENCE, D W J, and BLAKE, T. 1978. Estuarine sedimentation in the Eocene of southern England. Sedimentology, Vol. 25, 861-876. 
DEWEY, H, and BROMEHEAD, C E N. 1915. The geology of the country around Windsor and Chertsey. Memoir of the Geological Survey of Great Britain, Sheet 269 (England and Wales). 
HAWKINS, H L. 1954. The Eocene succession in the eastern part of the Enborne valley, on the borders of Berkshire and Hampshire. Quarterly Journal of the Geological Society, London, Vol. 110, 409-430. 
ELLISON, R A, and WILLIAMSON, I T. 1999. Geology of the Windsor and Bracknell district - a brief explanation of the geological map. Sheet Explanation of the British Geological Survey, 1:50 000 Sheet 269 Windsor (England and Wales). 
WARBURTON, H. 1822. III.—On the Bagshot Sand. Transactions of the Geological Society of London, Vol. Series 2, Volume 1, 48-52. 
PRESTWICH, J. 1847. On the main points of structure and the probable age of the Bagshot Sands, and on their presumed equivalents in Hampshire and France. Quarterly Journal of the Geological Society, London, Vol. 3, 378-409. 
CURRY, D. 1958. Part 3a XII Palaeogene. Lexique Stratigraphique International. WHITTARD, W F, and SIMPSON, S (editors). Vol.1 Europe (Paris: Centre Nationale de la Research Scientifique.) 
Judd, J W, 1882. On the relations of the Eocene and Oligocene strata in the Hampshire Basin. Quarterly Journal of the Geological Society, Vol.38, p.461-489. 
Curry, D, Adams, C G, Boulter, M C, Dilley, F C, Eames, F E, Funnell, B M and Wells, M K. 1978. A correlation of Tertiary rocks in the British Isles. Special Report of the Geological Society of London, No.12. 
KING, C. 1982. Comments on the nomenclature of the Claygate Beds and Bagshot Beds of London and Essex (Bristow, 1982) and the Claygate beds of Essex (Bristow, Ellison, & Wood, 1980). Tertiary Research, Vol. 4, 47-52. 
Bristow, C R, 1982. The nomenclature of the Bagshot Beds and Claygate Beds of London and Essex. Tertiary Research, Vol.4, p.7-8. 
King, C, 1981. The stratigraphy of the London Clay and associated deposits. Tertiary Research Special Paper No.6. (Backhuys: Rotterdam). 
ALDISS, D T. 2012. The stratigraphical framework for the Palaeogene successions of the London Basin, UK. British Geological Survey Open Report OR/12/004. Available from http://nora.nerc.ac.uk/17121/. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E240 E241 E256 E257 E258 E259 E266 E267 E268 E269 E270 E272 E273 E283 E284 E285 E286 E298 E314 E327
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